Avoid mongodb compatibility regression by using 'ping' instead of 'hello' command

[arm4b]

A follow-up to #5302 where we replaced isMaster with hello for checking Mongo connection. Instead of mongodb version-specific hello, use ping instead that works with any mongo version as a safer alternative.

See https://www.mongodb.com/community/forums/t/how-to-use-the-new-hello-interface-for-availability/116748/

Credit goes to @blag for finding a better solution.
@lukepatrick would appreciate your feedback as well if we're good here.